A sample is put under vacuum in order to perform sample observation using a scanning electron microscope and a transmission electron microscope, and thus the sample is desired to be resistant to vacuum and to be imparted with electrical conductivity required to acquire an image thereof.
The preparation of a sample for scanning electron microscope is performed as follows. The sample is dehydrated by vacuum drying in advance, and then an electrically conducting material (platinum, carbon, gold, palladium, osmium, and the like) is coated on the surface of the sample by a means of vapor deposition, sputtering or the like in order to impart electrical conductivity and improve the generation efficiency of secondary electrons.
Materials, such as metals and semiconductors, which have electrical conductivity and resistance to vacuum, do not require such a pretreatment, but materials, which do not have electrical conductivity, requires conductive film coating by an electrically conductive material. In addition, conductive film coating by an electrically conductive material is required for materials deteriorating in resistance to vacuum, that is, materials deformed by vacuum drying or irradiation of an electron beam in a vacuum at the time of electron microscopic observation, as well.
Biological/living body samples contain a large amount of water, and thus vacuum drying in advance is required in many cases. For this reason, the surface shape thereof is significantly deformed in some cases, and thus it is difficult to observe a biological/living body sample in the living state as it is using an electron microscope.
Water-retaining state of wet samples such as gelatinous materials and foods can be observed at room temperature using a low vacuum SEM or a cryo-SEM, an environmental SEM (ESEM), and the like. According to these techniques, not only wet samples but also untreated samples can be observed. However, a high vacuum is required in order to perform observation at a high magnification, and thus the sample is required to be resistant to vacuum or electrically conductive. For that reason, it is difficult to acquire an image in the living state as it is at a high magnification in the observation of a biological/living body sample, as well.
In recent years, electron microscopic observation methods using an ionic liquid have been suggested. Patent Literature 1 and Non Patent Literature 2 disclose that a wet sample is scanning electron microscopically observed using an ionic liquid. Non Patent Literature 2 discloses the application of the method on cells, as well. In addition, Patent Literature 2 discloses a method using the technique on ionic liquids in transmission electron microscopic observation.